The present invention relates to a method for making a catalyst composition, in particular a method for making a phosphonitrile halide catalyst composition.
Phosphonitrile halide compounds are well known in the art for use as catalysts in the preparation of organopolysiloxanes by polymerisation reactions. For example, GB patent specification 765 744 discloses the use of polymeric chlorides represented by the formula (PNCl2)n, wherein n is an integer of at least 3, most preferably 3 to 6, as preferred phosphonitrile halides for the polymerisation of liquid organosiloxanes, and GB patent specification 910 513 discloses phosphonitrile halide catalysts for use in a process for the manufacture of stabilized high viscosity polyorganosiloxane oils. Particularly preferred in the art as catalysts for polymerizing polyorganosiloxanes are those phosphonitrile halide compounds of the formula (X(PX2=N)aPX3)+ Xxe2x88x92 or the formula (X(PX2=N)aPX3)+ (PX6)xe2x88x92 wherein X is a halogen, preferably chlorine, and a is an integer of from 1 to 9, preferably 2. The most generally used method for making these latter phosphonitrile halide catalysts is disclosed in U.S. Pat. No. 3,839,388, which method comprises reacting phosphorous pentachloride with ammonium chloride in a high boiling point chlorinated solvent, for example trichlorobenzene, dichlorobenzene or 1,1,2,2-tetrachloroethane. However, use of such chlorinated solvents is becoming increasingly unacceptable due to their toxicity and ability to cause ozone depletion, a problem compounded with the fact that linear phosphonitrile halides have a very low solubility in most other common organic solvents. There is thus a need for an alternative route to linear phosphonitrile halides without the use of chlorinated solvents.
This invention is a method for making a phosphonitrile halide of a formula (X(PX2=N)aPX3))+ Axe2x88x92 wherein X is a halogen, A is selected from the group consisting of X and (PX6)xe2x88x92, and a is an integer of from 1 to 9, comprising a first step of reacting PX5 and a disilazane under cooling in an inert non-chlorinated solvent and a second step of heating the reaction mixture to at least 50xc2x0 C. for a time sufficient to produce the phosphonitrile halide. The present inventors have found this route to phosphonitrile halide compounds useful as catalysts for use in the preparation of organopolysiloxanes, which route does not involve the use of chorinated solvents.
According to the present invention there is provided a method for making a phosphonitrile halide of the formula (X(PX2=N)aPX3))+ Axe2x88x92 wherein X represents a halogen, A represents X or (PX6)xe2x88x92, and a is an integer of from 1 to 9, which method comprises a first step of reacting PX5 and a disilazane under cooling in an inert non-chlorinated solvent and a second step of heating the reaction mixture to at least 50xc2x0 C. for a time sufficient to produce the phosphonitrile halide.
The method of the present invention has an advantage in that it produces substantially only linear phosphonitrile halide species, compared to the prior art method of U.S. Pat. No. 3,839,388 which produces both linear and cyclic species. Cyclic phosphonitrile halides are substantially inactive as catalysts for polymerizing polyorganosiloxanes and are thus undesirable as reaction products. The method of the present invention produces a mixture of linear phosphonitrile halide species of the above formula, the majority of product having a=1 to 3 with minor amounts of higher species, i.e. a greater than 3. The most effective phosphonitrile halides as catalysts for polymerizing polyorganosiloxanes are those where a=2 or 3.
In the first step of the method of the present invention, the disilazane may be of the formula R3SiNHSiR3, wherein each group R is independently an alkyl group, preferably a C1-C6 alkyl group, more preferably a methyl group, an alkenyl group, for example a vinyl or hexenyl group, or a hydrogen atom. The preferred disilazane is hexamethyldisilazane.
In the first step of the present method, the reaction between PX5 and the disilazane is performed under cooling due to the exothermic nature of the reaction. Preferably, the reaction is performed at 0xc2x0 C. or below, for example by cooling with salt or solid CO2, or by using a refrigerated recirculating cooling bath. The inert non-chlorinated solvent may be any solvent which does not interfere with the reaction between PX5 and the disilazane, for example a non-functional solvent such as a hydrocarbon. Suitable solvents include toluene, heptane, xylene, and cyclohexane, with toluene being preferred.
In the second step of the method of the present invention, the reaction mixture prepared in the first step is heated at a temperature of 50xc2x0 C. or above, for example under solvent reflux. The time required to produce the phosphonitrile halide product depends upon the reaction conditions; however, typical conditions are heating under reflux of the solvent for 1 to 4 hours. The phosphonitrile halide product separates from the solvent allowing easy removal thereof from the reaction mixture.
To increase solubilization of phosphonitrile halides in organic solvents, ease of handling, and decrease moisture sensitivity, they may be further reacted with a cyclosiloxane of the formula ((CH3)2SiO)b, wherein b is an integer of from 3 to 6, preferably 3, to afford a linear compound of the formula Zxe2x80x94((CH3)2SiO)bxe2x80x94Z wherein Z is X or a group OPX2(NPX2)aNPX2xe2x80x94 and X and a are as defined above. See, for example, the method disclosed in EP 0305737. Suitable cyclosiloxanes, for example, include hexamethylcyclotrisiloxane or octamethylcyclotetrasiloxane. The cyclosiloxane may simply be mixed with the reaction mixture formed from performance of the first and second steps described above and heated, preferably under reflux of the solvent. The reaction produces a mixture of compounds, some of which have both end groups Z as X, some have both end groups as groups OPX2(NPX2)aNPX2xe2x80x94 and some have one endgroup as X and the other as OPX2(NPX2)aNPX2xe2x80x94. The catalytically active species have at least one phosphonitrile endgroup, i.e. at least one endgroup Z as OPX2(NPX2)aNPX2xe2x80x94. The phosphonitrile halide containing species are soluble in some organic solvents, for example, ethylene glycol diacetate, which particular solvent has the advantages of being inert towards the phosphonitrile halide containing species and being very soluble in siloxanes. The solution which results from dissolving said phosphonitrile halide containing species in ethylene glycol diacetate is stable over time and the dissolved phosphonitrile halide containing species can retain catalytic activity for several months. The above described reactions may be represented schematically as follows: 
wherein R, X, Z, a and b are as defined above.